1. Field of the Invention
The invention relates to magnesium hydroxide of the chemical formula Mg(OH).sub.2 with new characteristics and process for production thereof. The Mg(OH).sub.2 of this invention differs from known Mg(OH).sub.2 and exhibits superior performance and properties in such uses as fire retardants for thermoplastic synthetic resins and aqueous paints as well as other well known applications for conventionally available magnesium hydroxides.
2. Description of the Prior Art
It is generally known that for the use in thermoplastic materials the most favorable magnesium hydroxide, coated or uncoated with a surface active agent, is one which exhibits well developed crystallinity and is chemically inactive. Such magnesium hydroxide has small specific surface S.sub.BET, usually less than 20 to 25 m.sup.2 /g. It is equally important when incorporating such magnesium hydroxide in a thermoplastic material that secondary particles should be as small as possible and their size distribution should be as narrow as possible. Magnesium hydroxide without the required properties after incorporating in thermoplastic materials causes a considerable deterioration of their physical parameters, especially their flow and shape properties . Such deterioration creates local inhomogeneities in the thermoplastic materials which appear on the material surface as color patterns.
Conventional available magnesium hydroxides do not fulfil these requirements and usually have large specific surface S.sub.BET --up to 100 m.sup.2 /g. Most procedures leading to preparation of special types of magnesium hydroxide with small secondary particles and with small specific surface S.sub.BET start with preparation of large crystals. Known procedures are adding of small amounts of citric acid or its salts (JP 10,784/1958/) or removing calcium cations (JP 10,786/1958/), ensuring that large crystals are formed. A different procedure (EP 0 189 098 A2) leads to morphologically different magnesium hydroxide with spherical, not angular secondary particles of the average size 5-500 .mu.m. Use of the procedure of EP 0 365 347 results in secondary particles with large specific surface S.sub.BET of 20 to 50 m.sup.2 /g.
Magnesium hydroxides produced by the process described in U.S. Pat. Nos. 4,098,762 and 4,145,404 are characterized by crystallite size in the direction &lt;101&gt; greater than 800 .ANG. and a strain in the &lt;101&gt; direction not greater than 3.0.times.10.sup.-3. Secondary particles arising from these crystals, exhibit specific surface S.sub.BET less than 20 m.sup.2 /g.
A common disadvantage of the procedures leading to preparation of large crystals of magnesium hydroxide is the necessity to work with diluted solutions which causes large consumption of energy when the procedures are applied. A disadvantage of the procedure disclosed in the cited U.S. Pat. Nos. 4,098,762 and 4,145,404 consists in the formation of very large crystals (the crystallite size in the direction &lt;101&gt; is 800 to 10000 .ANG.) with undefined shape, e.g. they may have an arbitrary ratio of crystal base width to crystal height (crystal size in the direction &lt;101&gt; does not define the crystal shape). In this way crystals may form which are thin and high or thick and low, but also crystals with approximately comparable thickness and height. Aggregation of such nonuniform crystal shapes leads to formation of secondary particles with higher specific surface m.sup.2 /g. The above mentioned disadvantage must be compensated for by preparation of large crystals (examples 1 to 5 U.S. Pat. No. 4,145,404 which report the crystallite size in the direction &lt;101&gt; as up to 2250-5260 A.degree.).